They are made from enriched uranium oxide . This contains
approximately 3-4% U-235. The fuel is then converted into small
pellets, packed into tubes called pins. The pins are bundled into
larger tubes and are ready to be put into the reactor.

Moderators

Moderators slow down the neutrons. Materials used
for moderators include graphite and heavy water, containing the
isotope deuterium (H-2).

Control rods

The core also has neutron-absorbing control rods to
control the neutrons released. While the moderator promotes stronger
interaction of the neutrons with the uranium nuclei, the control
rods are there to reduce the concentration of neutrons whenever
necessary.

The material used for these control rods is cadmium
whose nuclei have a large cross-section for the capture of neutrons.
The cadmium rods are inserted into the nuclear reactor to slow down
or shut down the chain reaction whereas withdrawing the rods
restarts or speeds up the reaction.

Coolants

The temperature of the reactor is very high although
the actual temperature depends on the reactor type. Some reactors
operate at temperatures as high as 1500 °C. The coolant flows around
the core and carries heat energy to a steam generator. The coolant
materials vary between reactors. They may be water or carbon dioxide
at high pressures. Some reactors such as fast breeders use liquid
sodium as coolant.

Generation of electricity

The coolant carries the heat energy to a heat
exchanger (or steam generator), where it is used to make steam. The
steam then drives the turbine, and the turbine drives the generator
that produces electricity. Waste steam goes to the cooling towers to
be condensed back into water.